In the manufacturing of integrated circuits, contact plugs are used for connecting to the source and drain regions and the gates of transistors. The source/drain contact plugs were typically connected to source/drain silicide regions, whose formation includes forming contact openings to expose source/drain regions, depositing a metal layer, performing an anneal to react the metal layer with the source/drain regions, filling tungsten into the remaining contact opening, and performing a Chemical Mechanical Polish (CMP) to remove excess tungsten. A cleaning is then performed. In the CMP and the subsequent cleaning processes, the top surface of the contact plug may suffer from dishing and corrosion.